1. Field of the Invention
This invention relates to a process for producing a vinyl chloride polymer while controlling formation of scales, and a vinyl chloride polymer obtained by such a process.
2. Description of the Prior Art
Processes in which vinyl chloride monomers or mixtures of vinyl chloride monomers and other vinyl monomers are subjected to suspension polymerization or emulsion polymerization in aqueous mediums in the presence of polymerization catalysts have the problems that polymers in the form of scales may adhere to inner surfaces of polymerization vessels, i.e., inner walls of polymerization vessels or other fittings of polymerization vessels at their portions with which monomers come into contact in the course of polymerization, e.g., the surfaces of stirrers or the like. Adhesion of such polymer scales to the inner surfaces of polymerization vessels may cause a decrease in heat transfer efficiency of polymerization vessel walls and in polymerization yield, and also the scales adhering thereto may peel from the inner surfaces of polymerization vessels to mix in products to cause what is called fish eyes, resulting in a lowering of the quality of products. Removing such scales also takes much labor and time to bring about undesirable many disadvantages such as a decrease in operating efficiency of polymerization vessels.
Especially in the case of suspension polymerization, the scales adhering to polymerization vessels have a lower porosity (porousness) than normal polymers simultaneously produced, and hence the scales having peeled therefrom and mixed in normal polymers make the polymers poorly compatible with molding additives including plasticizers added when the polymers are molded, so that particles not mutually melt in final products in part (what is called fish eyes) may occur. Also, if the polymerization is repeated without removing the scales from polymerization vessels, the scales may accumulate to even deform the inside shapes of polymerization vessels. Especially when the scales have accumulated on stirrers in a large quantity, they may greatly affect the whole suspension system to affect the shape, particle size distribution, yield and so forth of the resulting polymer particles.
For these reasons, it is indispensable to periodically remove scales from the inner surfaces of polymerization vessels. To remove them, it is necessary to make the operation of polymerization vessels stop or pause, resulting also in an increase in the cost not directly concerned with the manufacture of polymers. The scales are commonly removed by scraping them from the inner surfaces by a mechanical means, making a high-speed water jet strike against the scales, or dissolving them with organic solvents. In some part of such operations, operators must enter the vessels to make operations, where they are involved in a possibility of touching the vinyl chloride monomers that are considered causative of cancer, which is very undesirable. Also, removing the scales from polymerization vessels by a mechanical means has a high possibility of damaging the inner surfaces, and this may undesirably promote the formation of scales in the subsequent polymerization. Besides, the method of removing the scales by the use of solvents additionally requires equipment for the recovery of solvents and cost and expenses for the driving of such equipment.
Accordingly, as methods for preventing adhesion of the polymer scales, a method is known in which the inner walls of polymerization vessels are coated with a scale preventive agent to form coating films thereon, and various scale preventive agents are proposed. For example, dyes or pigments (Japanese Patent Publication No. 45-30835), polar organic compounds (Japanese Patent Publication No. 45-30343), aromatic polyamine compounds (Japanese Unexamined Patent Publication No. 53-23381), condensates of phenol compounds with formalin or benzaldehyde (Japanese Unexamined Patent Publication No. 54-36389) and so forth are proposed as the scale preventive agents.
The coating films of these scale preventive agents are formed in the following way: A polymer slurry formed in a polymerization vessel is drawn out of it after polymerization is completed, and then the inside of the polymerization vessel is cleaned with water, followed by three steps of (1) spray-coating a polymer scale preventive agent coating solution on the inner walls of the polymerization vessel and the other portions with which vinyl monomers come into contact in the course of polymerization, from spray nozzles provided at the top of a vapor phase zone inside the polymerization vessel (a coating step), (2) drying the coating surfaces after the spray coating to obtain dried coating films (a drying step) and (3) washing the dried coating films with water (a washing step).
After coating films of the scale preventive agent are formed in this way, vinyl chloride monomers and other materials are charged into the polymerization vessel to carry out polymerization to produce vinyl chloride polymers. In recent years, in order to improve productivity, it is desired to shorten the cycling time of the process comprising a series of steps of the steps of forming the polymer scale preventive coating films and charging the materials, the step of polymerization reaction, the steps of collecting unreacted monomers and drawing out the polymer slurry and the step of washing the inside of the polymerization vessel with water. As a part of such desire, it is also desired to shorten the time taken for forming the coating films.
At the same time, a polymerization process is recently employed which makes use of a large-size polymerization vessel having a capacity of 40 m.sup.3 or more for the purpose of improving productivity and also can shorten the polymerization reaction time. In such a polymerization process that can shorten the polymerization time, a method in which a polymerization initiator is used in a large amount is employed. Use of the polymerization initiator in a large amount results in an increase in heat values per unit time, and hence it becomes necessary to improve heat elimination efficiency. A heat elimination means is typified by a cooling jacket and a reflux condenser. In the case of the large-size polymerization vessel having a capacity of 40 m.sup.3 or more, the jacket is insufficient for the heat elimination, and it becomes necessary to make the rate of heat elimination higher by the use of the reflux condenser. However, with an increase in the load of heat elimination by the reflux condenser, the slurry inside the polymerization vessel may more bubble and the slurry may overflow into the reflux condenser, so that the heat elimination capacity of the reflux condenser may lower or the polymer scales may adhere to the inside of the reflux condenser. In some instances, polymer particles overflowed may cause pipes to clog to make it impossible to operate the reflux condenser. Moreover, if the reflux condenser is used from the beginning of polymerization, problems may arise such that the resulting polymer particles have coarse particle size and foamy polymers are produced.
Accordingly, in the case when such a large-sized polymerization vessel is used to shorten the polymerization reaction time, the heat elimination capacity may necessarily be insufficient when only the jacket and the reflux condenser are used, and the insufficient heat elimination capacity must be compensated with any additional cooling means. As such additional cooling means, cooling devices such as a cooling coil, a draft tube and a cooling cylindrical baffle are provided in the polymerization vessel.
Now, when the coating films of the polymer scale preventive agent are formed, the spray coating that can be carried out in a good efficiency as stated above is employed. In this spray coating, however, the coating solution can not be well applied on the portions shadowed as viewed from the spray nozzles inside the polymerization vessel, e.g., the surfaces of baffles provided in the polymerization vessel, facing the inner wall of the polymerization vessel, and hence the coating may become uneven. Especially when the inside of the polymerization vessel has a complicated structure because of the internal cooling devices, the coating may become insufficient at many portions. Hence, with an increase in the number of repeating polymerization batches, the polymer scales may more adhere to the portions where no sufficient coating films are formed. As a countermeasure therefor, the coating solution is well sprayed so as to cause no uneven coating, i.e., the coating solution of the polymer scale preventive agent is applied in a large quantity over a long time in the coating step. As the result, however, the drying time required in the drying step often must be made longer with an increase in the quantity of the coating solution, and also, since the coating solution of the polymer scale preventive agent remains in a large quantity in the washing step, the washing time must be made longer in order to remove it.
To make longer the time taken for forming the coating films in this way has brought about the disadvantage that it goes against the above requirement for making the polymerization time shorter.
In addition, the use of the polymer scale preventive agent coating solution in a large quantity may bring about ill effects such that it greatly causes initial discoloration of the vinyl chloride polymer products obtained by polymerization and causes an increase in colored foreign matter ascribable to the scale preventive agent included in the products.